Non-linear optical effect of a crystal is an effect that the frequency of the laser beam will be modified when a laser beam with certain polarization and incidence direction passes through the non-linear optical crystal (such as MBBF). The typical sketches of this effect are shown in FIG. 1 and FIG. 2.
The crystals with non-linear optical effect are named non-linear optical crystals. The non-linear optical effect herein implies the effect such as second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), optical parametric oscillation (OPO), optical parametric amplification (OPA) and the like. Only those crystals without inversion center might exhibit non-linear optical effect. Using the non-linear optical effect of, the crystals, the non-linear optical devices can be constructed, for example, second harmonic generator, sum\difference frequency transformer, optical parametric oscillator and etc. The laser generated from the laser generator can achieve frequency transformation by the non-linear optical devices. For example, an infrared laser beam (e.g. 1064 nm) can be transformed to the spectrum region of visible, UV or even deep-UV (with the wavelength less than 200 nm) lasers through the non-linear optical crystals. Thus the non-linear optical crystals have great potential in the laser technique field. Nowadays, there are three kinds of inorganic non-linear optical crystals which are the most widely used ones in this wavelength region, i.e. the low temperature phase of barium meta-borate (β-BaB2O4, abbv. BBO), lithium triborate (LiB3O5 abbv. LBO) and potassium titaniol phosphate (KTiOPO4, abbv. KTP). However, the effective SHG output wavelengths of the three crystals mentioned above have some limitations in the UV spectrum region. For BBO, it is because that (1) (B3O6) group with large conjugated π orbital characteristics allows for the red shift of the band gap of the group, which results in the absorption edge of BBO crystals at 189 nm; (2) due to the limitation of the UV absorption edge, the crystals are not capable to generate harmonic laser with wavelength less than 193 nm; (3) the double refractive index of the BBO is Δn≈0.12 resulted from the planar (B3O6) group, and the large double refractive index leads to an acceptance angle of Δθ=0.45 mrad at fourth harmonic generation, which is too small for the practical uses of the devices. For LBO, the double refractive index is too small to achieve phase matching at short wavelength, therefore it is not able to achieve the effective SHG output in the same region. As to KTP, the UV cut-off is 350 nm, thus it is also not capable to generate UV harmonic laser. [BBO(β-BaB2O4), see Science in China B28, 235, 1985; LBO(LiB3O5) crystals, see Patents for Inventions in China 88102084; KTP (KTiOPO4), Handbook of Nonlinear Optical crystals]. 
So far, KBe2BO3F2 (abbv. KBBF) is the only non-linear optical crystal which is capable for direct SHG output in the deep UV region. This crystal is invented and developed by R&D Center for Crystals, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, a group under Academician C. T. Chen's leadership.
The KBBF crystal is constructed with planar triangular (BO3) group and tetrahedral (BeO3F) group, wherein the three O atoms of (BO3) group connect to Be atom respectively to form a 2-dimensional infinite net and the K+ ions locate between the planes, which interconnect by electrostatic force. Non-linear optical effect of the crystal is mainly contributed by (BO3) group. The (BO3) groups arrange in planes in the crystal lattice and they parallel with each other and are vertical to the crystal c axis, which allows for the superior non-linear optical properties of the crystal. The absorption edge of this crystal is 155 nm, the double refractive index thereof is about 0.07, and the phase matching region can be expanded to 170 nm. With prism-coupling technique, the KBBF crystal has achieved fifth harmonic generation output of the Ti:sapphire laser (157 nm-160 nm) [see J. Opt. Soc. Am. B (2004), 21(2)].